Glass texture has Voronoi like texture on it when using glass BSDF
The model is a mess. When you download models created by other people you are always prone to inherit a lot of buggy geometry and bad modelling habits of the creator, plus those created by the translation into different formats.

The particular model is plagued with duplicated geometry, hence the z-fighting. All of those dark triangles that you see are not only on the windshield but in every surface, are caused by overlapping vertices

You need to do "remove doubles".

The normals are all messed up (in edit mode select all of the vertices and make them consistent (Ctrl+N).

On top of that, whenever you are dealing with glass you need it to have some thickness (see this link: https://blender.stackexchange.com/a/69616/1853)

The glass shader you are using makes no sense. See this link for a proper  shader: How to illuminate the darkness inside glass objects in Cycles?. 

The scale for the object is also wrong.

There is probably more, but that you will have to figure out on your own.

If you are doing experiments with textures, first make sure that the model you are going to use is decently crafted so that you can focus on the textures.